1. Field of the Invention
The present invention relates to an optical regenerative repeater, particularly to an optical repeater preferably used for a digital optical transmission system.
2. Description of the Related Art
In the case of a signal transmitted through a transmission line, the waveform is distorted as noises or jitters are accumulated. When these analog deteriorations accumulate in an optical transmission signal, errors are increased to a serious level. Therefore, a signal regenerative repeater is used to receive a signal, discriminate the symbol and transmit the signal before errors accumulate to such a level.
The function of the regeneration repeater roughly comprises reshaping for compensating attenuation and waveform distortion, retiming for removing jitters, and discriminating (regenerating) for re-discriminating the symbol and removing noises. These are referred to as the "3R function". Some of the above analog signal-waveform deteriorations are fixed as permissible symbol errors and most of them are removed by the 3R function. In the case of a present optical transmission system, every processing for the equalizing amplification can optically be made by an optical amplifier. However, recovery of deteriorations other than the processing for the equalizing amplification is performed by an electrical circuit.
In a general regenerative repeater, the transmission of electric signals (input signals) are branched to two parts, or pathways. A clock regenerator regenerates a clock signal from the branched transmission signal. The logical product (the AND logical operator) between a regenerated clock signal and the other branched transmission signal is taken by an intensity modulator to obtain a regeneration signal (output signal).
Japanese Laid-Open Patent Application No. 36581/90 also discloses an optical shaping device. In the device, the input optical signal is branched to two paths, one branched optical signal is input to an optical semiconductor laser amplifier, and another branched optical signal is converted to an electrical signal to extract the clock signal. The clock signal is superimposed on a driving current of the optical semiconductor laser amplifier.
As described above, a conventional regenerative repeater uses an electrical circuit for signal regeneration. However, it would be ideal if the optical signal could directly be regenerated. This is because the transmission signal is light and the regenerative repeater using the electrical circuit has a structure in which a receiver and a transmitter are arranged back to back. Therefore, the structure unavoidably becomes complex and expensive. Optical signal processing would make it possible to greatly simplify the structure and decrease the cost.
Also for the operation speed, the upper limit of the above-described electrical circuit processing is approximately 20 GHz and further acceleration of the operation speed is inherently limited. In the case of a transmitter and a receiver using optical signal processing, however, operation speeds up to approximately 1,000 GHz (1 Thz) have been demonstrated. Therefore, when the restrictions on the operation speed of an electrical circuit of a regenerative repeater have been removed, it can be expected that the transmission speed of a regenerative repeater system can greatly be improved.